The present invention relates to ink compositions suitable for thermal inkjet printing, and, more particularly, to ink compositions that are film forming and provide improved drop-velocity stability and prolonging resistor life in inkjet pens.
The use of inkjet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today""s inkjet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality, while further lowering cost to the consumer.
An inkjet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a xe2x80x9cprintheadxe2x80x9d onto a printing medium. The typical inkjet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a xe2x80x9cfiring resistor,xe2x80x9d located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 xcexcmxc3x9735 xcexcm. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., inkjet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
The small length scale of the nozzles, typically 10 to 40 xcexcm in diameter, require that the ink not clog the nozzles. Further, repeated firings of the resistor elements that must withstand many millions of firings over the life of the ink cartridge to be commercially practical, can result in fouling of the resistor elements and degrading pen performance. This build up of residue on the resistor elements is unique to thermal inkjet printers and is known as kogation and defined as the build-up of residue (koga) on the resistor surface.
Besides the problem of kogation, firing resistor surfaces are susceptible to passivation layer damage by cavitation, contamination and many other sources. Such passivation layer damage literally results in microscopic holes on the resistor surface which significantly shorten resistor life. Energizing of the firing resistor after hundreds of millions or even tens of billions of times can erode away the top passivation layer, which is typically tantalum. This erosion may be from a combination of oxidation, chemical attack by the ink at high temperatures, and cavitation.
Erosion of the top passivation layer can lead to the failure of the underlying electrically insulating layers, causing the circuit which provides power to the resistor to short out. If the electrically insulating layers are not compromised, erosion can degrade drop velocity stability by adversely affecting the heat conduction properties of the resistor.
Minimizing drop-velocity variations between nozzles and within nozzles is critical for accurate drop placement on paper. Drop placement errors degrade both text and image quality. The magnitudes of the placement errors caused by velocity variations are dependent on pen-to-paper spacing and pen scanning speed relative to the paper. Therefore, as thermal inkjet printers become faster and print on a greater variety of media, greater pen-to-paper distances will be needed and it will become more important to decrease drop velocity variations. Furthermore, drop placement errors are more noticeable with small drop-volume pens; the smaller drops cannot mask the errors.
Drop velocity variations are thought to be due to a combination of erratic drive bubble nucleation and variations in energies delivered to each resistor. The former may be more important for velocity variations within a given nozzle. The latter may be more important for velocity variations between nozzles and can be due to different resistances through the electrical traces between the power supply and each resistor. These parasitic resistances result in slightly different amounts of power being delivered to each resistor. Erratic drive-bubble nucleation can be due to surface roughness or pits on the resistor surface that provide low energy nucleation sites. Koga, a carbonaceous film formed from thermal decomposition of organic components in the ink, can especially contribute to surface roughness. Also, erratic bubble nucleation may be caused by sharp temperature gradients on the resistor surface that may cause nucleation to occur first over the center hot spot of the surface of resistor as opposed to a uniform nucleation over a greater fraction of the resistor surface area. The problem of sharp temperature gradients is worse in small drop volume pens. In addition, sharp temperature gradients can lead to local high temperatures on the resistor. Higher resistor temperatures worsen kogation build up. This rough carbonaceous deposit provides many nucleation sites, leading to early, erratic vapor-drive bubble formation, low drop velocity and drop weights.
Customer and profit demands require smaller drop volumes, color-laser-like ink permanence, and xe2x80x9cpermanentxe2x80x9d print heads. Smaller drop volumes give better spatial and chroma resolutions. However, passivation layer damage appears to be worse in smaller drop volume pens. In small drop volume pens each resistor must fire a greater number of times to transfer the same amount of ink to the page. The greater number of firings required of the resistor results in more passivation layer damage.
Reducing passivation layer damage by increasing the passivation layer thickness is typically not practical in high throughput printers. Resistors with thicker passivation layers require more energy to eject an ink drop. However, most of this excess energy is retained as heat within the passivation layer and is not effectively transferred to the ink. Therefore the power requirements are greater and more expensive printer components may be needed. Furthermore, this retained heat can build up in the thermal inkjet pens that would cause the pens to overheat. Printing speeds would need to be reduced or elaborate cooling schemes employed to avoid the overheating.
The trend is towards longer print-head life, using pens with replaceable ink supplies such as (but not limited to) off-axis ink reservoirs that are connected to the pens by hoses and ink reservoirs that snap onto the print head. Infrequent need for replacement of the print heads with prolonged resistor life reduces the cost and servicing required of the customer. High-speed, high-throughput photocopier-like products that may be envisioned for the future will greatly increase ink usage and will most likely greatly push further the demands on print-head life. With higher pen-to-paper relative speeds, high-throughput products will be more sensitive to passivation layer damage induced drop velocity variations.
Even though some kogation and/or passivation layer damage control methods in inkjet ink pens are known, all of them are either limited in their effectiveness, are not economically feasible or have undesirable side effects for pens needing long resistor life. Thus, there is even more of a need to find a way to effectively deal with the problem of passivation layer damage on inkjet resistors.
Currently, tantalum is typically used as the material in the top coat film of the resistor. The metal is very hard and is resistant to cavitation damage. The metal has good chemical resistance. In spite of the beneficial properties of tantalum, the topcoat can erode after repeated firings many hundreds of millions or even tens of billions of times. In addition, defects in the tantalum can degrade the uniform nucleation properties of the surface and, as a consequence, diminish print and image quality.
What is needed is a way of renewing the surface on the top of the resistor during the repeated firings of the resistor. The renewal of the surface should be able to fill in pits and defects on the top coat and provide a more uniform nucleation surface with a more uniform temperature distribution during firing.
The film may not necessarily be hard if it can be renewed at a sufficient rate. An analogy can be made with erosion of the shoreline. The tantalum is like a granite cliff that eventually erodes from the action of the waves. The ceramic film of this invention is like a sand beach. Though the sand is easily moved and eroded by the waves, the beach will continue to exist as long as there is a sufficient supply of sand from the neighboring beach or from a nearby river. In the case of the thermal inkjet resistor, a continual supply of metal ions for the film formation comes from the ink itself.
Due to the added passivation of the renewable surface, the tantalum layer thickness can be thinned, minimizing the heat retained in the tantalum top coat. By providing a surface coating derived from the interaction of the ink with the resistor, it will be possible to substitute the tantalum top coat with a less durable material including silicon, silicon oxide, silicon nitride, and silicon carbide.
It has been previously disclosed that film formation may benefit resistor life. In the background of a patent on chelates for kogation control, Aoki and Koike disclose the possibility of beneficial film formation as disclosed in Japanese Patent Application Laid-open No. 56042684, with a substance used as a film-forming means in the ink to form a film on the surface of the heater. This surface film can relieve the shock to the surface that occurs at the time of generation and extinction of bubbles (cavitation). Substances that can be used as film-forming means can include metal-containing compounds such as organic metal chelate compounds, metals of an organic acid, metallized dyes and the like.
It has been previously disclosed that high amounts of aluminum salts can minimize black-to-color bleed and improve waterfastness. The addition of 1 to 10 wt % of aluminum chloride and other multivalent salts to cationic-dye inks was patented by Stoffel for black-to-color bleed control. Hackleman later patented a method for increasing waterfastness by reacting anionic dye with aluminum chloride either in the media or deposited on the media with a xe2x80x9cfifth penxe2x80x9d with a 2 wt % aluminum chloride concentration.
At present, there is no patent literature concerning the addition of small amounts of aluminum salts to ink comprised of anionic dyes, especially aluminum salt additions that lead to film formation on the thermal inkjet resistor surface. Here xe2x80x9csmall amountsxe2x80x9d is meant below 1000 parts-per-million (ppm). Furthermore there is no mention of ink-derived films improving thermal inkjet drop stability.
The present invention relates to a method of forming a metal oxide film on a surface of a thermal inkjet resistor to reduce kogation and prolong inkjet pen life comprising firing the resistor at least one time to inkjet print an image on a medium with inkjet ink, wherein the ink comprises: at least one colorant; and an aqueous vehicle, the vehicle comprising aluminum ion in an amount sufficient, when the composition is used in an inkjet pen, to form a protective thin layer on an outer layer of a resistor surface of the inkjet pen, the outer layer comprising a refractory metal, a noble metal, a silicon composition or mixtures thereof, the refractory or noble metal being selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, gold, silver, platinum, silica, silicon, silicon nitride, silicon carbide and mixtures thereof.
The present invention also relates to a thermal inkjet printhead comprising alumina-coated resistors.
Additionally, the present invention relates to a thermal inkjet ink comprising:
at least one colorant; and an aqueous vehicle, the vehicle comprising
aluminum ion in an amount sufficient, when the composition is used in an inkjet pen, to form a protective thin layer on an outer layer of a resistor surface of the inkjet pen, the outer layer comprising a refractory metal, a noble metal, a silicon composition or mixtures thereof, the refractory or noble metal being selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, gold, silver, platinum, silica, silicon, silicon nitride, silicon carbide and mixtures thereof.
Furthermore, the present invention relates to a method for inkjet printing, said method comprising the step of ejecting ink, said ink comprising: at least one colorant; and an aqueous vehicle, the vehicle comprising at least one refractory or noble metal-reactive component in an amount sufficient, when the composition is used in an inkjet pen, to form a protective thin layer on an outer layer of a resistor surface of the inkjet pen, the outer layer comprising a refractory metal, a noble metal, a silicon composition or mixtures thereof, the refractory or noble metal being selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, gold, silver, platinum, silica, silicon, silicon nitride, silicon carbide and mixtures thereof.